A hydraulic power-steering or power-assist steering system has a valve operated by the control member, the steering wheel, and connected between a source of hydraulic liquid under pressure and the control member, normally a power cylinder connected to the vehicle wheels. This power cylinder is a standard piston-and-cylinder unit having a piston rod extending through one wall of the cylinder, so that the effective surface area of the piston exposed in one compartment of the cylinder is smaller than that exposed in the other. In the intermediate position of the control valve both sides of the piston are equally pressurized. As a result of the difference in effective surface areas a net force is generated which urges the piston in one direction. This has the result of steering the vehicle to one side.
It has been suggested to overcome this disadvantage by providing a counteracting spring inside the power cylinder. Such an arrangement has, however, been found relatively ineffective due to the varying spring characteristic at different degrees of compression. Only in a central range does the spring force accurately counteract the differences in pressures on the piston, whereas at both ends, that is at maximum compression and at minimum compression, the spring force is different and the above-described imbalance in the system is present.